Magnetic tape recording of typewriter keyboard data



Sept. 24, 1968 MAGNETIC TAPE RECORDING OF TYPEWRITER KEYBOARD DATA J. D.MISLAN Original Filed Nov. 14, 1962 SEND Esc.

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A (50g 0 U 28j-fwn O C United States Patent O MAGNETIC TAPE RECORDING 0FTYPEWRITER KEYBOARD DATA Joseph D. Mislan, Morrisville, Pa., assignor toCommunitype Corporation, a corporation of Delaware Continuation ofapplication Ser. No. 237,598, Nov. 14, 1962. This application Dec. 7,1967, Ser. No. 688,937 6 Claims. (Cl. 179-2) ABSTRACT 0F THE DISCLOSUREAn inter-office data communication system station comprises an othcetypewriter with input-output facilities and electronic circuitry linkingthe machine either directly to a tone-code receiver-transmitter andthereby to a standard voice telephone channel, or optionally via amagnetic tape recorder-producer for high-speed bitparallel, word-serialstorage and retrieval of typewriter keyboard operations. Provision ismade for code parity checking, tape backspacing for error correctionunder keyboard control, local typing from tape records, error counting,and for various operation modes for text and data transmission, computerterminal use, and other applications.

This is a continuation of application Ser` No. 237,598, filed Nov. I4,1962, and now abandoned.

This invention has to do with improvements in communication systems ofthe type in which data or information of the kind which can be encodedby a keyboard, such as a typewriter keyboard, is recorded as codeSequences upon a suitable recording medium; the recording being at asuitable time played-back or reproduced over a transmission channel to areception point. At the latter point, the incoming code sequences areeither again recorded for subsequent control of a typewriter at thatstation, or can optionally be applied directly to a typewriter or to theinput of other suitable data-processing equipment such as a computer.

It is a principal object of the invention to provide an improved systemand equipment of this general type which will make the fullest use ofmodern data-handling technique by electronic means, and which willprovide reliable interofhce data handling communication, and auxiliaryfeatures useful therewith, at relatively low cost considering theresults achieved.

It is recognized that intercommunication by means of coded pulsesgenerated by the operation of keyboards associated with page printertelegraph apparatus is accomplished by known teletypewriter systems, andthat such known systems also include code-recording and reproducingauxiliaries which enable the storage of signals for interofiicetransmission (and reception) according to the exigencies of trafficconditions, or to suit the needs of users. While the present inventionis in a sense directed to the satisfaction of the same general orultimate requirements, it is characterized by differences both as todetail and as to operating principles and features which in combination,or cooperatively, provide extensive and important advantages. Theselatter will best be understood from the following specific objects ofthe invention, which are so drawn as to provide a comparison withgenerally analogous prior-art systems so as to avoid a tediousrepetition of the history of such prior art.

It is accordingly one specific object of the invention ro provide asystem of the kind mentioned above in which the inter-officecommunication link can be a conventional local or long-distance voicetelephone circuit, rather than a specially designed and maintainedtelegraph circuit. The satisfaction of this object effects majoreconomies and 'lll 3,403,225 Patented Sept. 24, 1968 conveniences ininstallation and operation, not the least important of which is thatsaid voice telephone channel or circuit remains available for speechcommunication whenever it is not in use for code transmission. Theparticular exemplary embodiment of the invention described below, forexample, utilizes an existing code-transmission system of the typedesignated by the trademark Dataphone," and described in the March 1962issue of the Bell Laboratories Record.

A further specic object of the invention is to provide such a system inwhich the page-printing keyboard machine ultimately used as the codegenerating (and optionally as the receiving) device may be in essence amodern form of the ordinary electric office typewriter sold under thetrade name Selectric," provided with inputoutput features which simplifythe keyboard selection of codes and sequences for transmission to otherlocations, and which also simplify their use in control of the pageprinting or typing operations at the said remote points. The codesutilized by the invention include not only the codes for letters,characters and symbols as provided in code `form by the Selectricmachine of the inputoutput type, but include also codes corresponding tononprinting functions of the machine, including the selection of upperand lower case, backspacing, forward or carrier spacing, tabulating, andcarrier returning, The commercial typewriter of the kind mentioned doesnot directly provide these codes, and the invention therefore providesauxiliary means controlled by the corresponding keyoperated contacts orswitches of the machine, for generating such function codes.

Still another object of the invention is to provide, in such a system,simple code storing and retransmitting equipment uitilizing magnetictape as the storage medium both for the temperorary storage of locallygenerated codes pending their later transmission over the telephonecircuit, and for the interim storage of received codes pending theirtranslation into forms suitable for data processing or to produceprinted text by the typewriter, as described, at the receiving ofiice.More specifically, the invention comprehends a system in which themagnetic tape steps a discrete amount to record each code bit of eachselected or command code, the bits being recorded on the tape in serialorder, and each code or sequence of bits normally comprising eight bits.Six of the bits will define the coded character or function, one bitwill provide a parity check facility, and these will be preceded by astart bit. No stop pulse is required, and the system permits much higherrates of transmission than are possible with somewhat similar start-stopsystems ernployed with previously known punched-tape perforators andreperforators of known typewriter systems with their associated noise,mechanical complexity, requirements for a continuous supply of bulkyone-use paper tapes, special operator training, and so on.

An object of the invention which is directly related to the foregoing isthe provision of a facility by which an operator preparing a magnetictape record of typewriter operations may readily correcterroneously-selected codes, to the end that the message may later betransmitted accurately and at maximum speed; e.g., without the waste oftransmission time associated with the presence of numerous delete codesas in known paper-tape systems.

A further object related to the two immediately preceding paragraphs isthe provision of multipurpose transistorized data converting circuitsconnected between the typewriter and the recorder-producer and betweenthe recorder-reproducer and the voice-coded signal transmitter andreceiver, to effect the necessary signal conversion operations such asparallel-to-serial conversions and the like, and which also rendercompatible the timing characteristics of the various components. Amongthese latter are (1) the essentially random timing of manual typewriterkeyboard operations, (2) the divergent operatingtime requirements of thetypewriter as between characterprinting operations and purely functionaloperations such as line feeding, case-shifting and the like, (3) thedivergent limits upon cycle repetition rates dictated by the design ofthe magnetic tape recorder-reproducer and the telephone line datasystem, and others.

Yet another object of the invention is to yprovide a system as above inwhich the aggregate of auxiliary equipment is minimized by theutilization of portions thereof to satisfy multiple or differentfunctions in accordance with the immediate operating requirements: Forexample, the accomplishment of this object is contributed to in animportant way by the employment of a magnetic type recorder-reproducerwhich is selectively used (l) `for the indefinite storage oflocally-generated code sequences pending their later transmission to aremote station, and used (2) for the indefinite storage of codesequences arriving from a remote station pending their later use incontrolling the local typewriter to `produce printed copy.

In brief, the invention provides for the interconnection over standardtelephone circuits of office-style typewriters without interfering withthe other intended or useful functions of such circuits and machines,and essentially without imposing special requirements thereon. Theinvention further provides, in such a combination, novel facilities forcode storage and reproduction which overcome the limitations of priordesigns directed to similar ends.

Finally, the invention provides simple, quiet, compact,

relatively inexpensive and highly reliable auxiliaries to accomplish thestated objects, making maximum use of solid-state electronicconfigurations and magnetic code recording and reproducing facilities.

A complete embodiment of a preferred form of the invention will bedescribed below by way of example and illustration, so as to enablethose skilled in the art to practice the invention, but withoutintention to thereby limit the full scope of the same as defined in theclaims appearing at the end of this specification.

In the drawings:

FIG. 1 is a simplified block diagram of the chosen embodiment of acomplete system.

FIG. 2 is a similar diagram, with additional details, of the equipmentof one station only of the system.

FIG. 3 is a still more detailed diagram of the transmitting portions ofsuch a single station.

FIG. 4 is a diagram Similar to FIG. 3 showing the receiving functions ofthe station.

FIG. 5 is a diagram detailing the construction and operation of thecorrection circuit for the magnetic-tape recording and reproducingequipment of one station.

FIG. 6 is a schematic representation of a section of the record tape,illustrating the positioning of the code sequences thereon.

General Referring first to FIG. 1 of the drawings, a typical two-stationsystem is indicated functionally in block form, for two-waycommunication. The telephone circuit is designated by numeral 10, theapparatus to the left thereof being located at one station, and that tothe right being at the other. Only the left station is shown in anydetail, since the station equipments will be duplicates where two-waycommunication is desired. Otherwise, one station will require onlytransmitting equipment and the other only receiving equipment.

The equipment at the left station (Station #1) in FIG. l comprisesmainly the Selectric typewriter 12 with the usual keyboard 14 andprinter mechanism 16, typically as shown in U.S. Patent 2,879,876 ofPalmer et al. Other suitable typewriters can be employed, and in anyevent are to include input-output auxiliaries (which are standardcommercial equipment) indicated as a set of electrical code contacts 18operated by each character key and a single contact operated by eachfunction key. Such auxiliaries also include a set of printer operatingsolenoids or the like indicated at 20. Operation of the keyboard printstext as usual on paper inserted in the typewriter, and, when thestation-to-station equipment is in use, the keyboard operated contactssupply current over conductors 22 (in the form of a six-bit code plusone parity check bit) to an encoder apparatus 24 to produce, at itsoutput 26, final eight-bit codes (including for each the start pulsebit) corresponding to the respective keyboard operations. These 8-bitcodes are recorded serially, on a magnetic tape, by tape recorder 28associated with magnetic tape reproducer 30.

At any time after a sufficient number of codes sequences have beenrecorded, or when actual transmission to Station #2 is desired, therecorded tape is read" (that is, its signals are reproduced) byreproducer 30, whose pulse output (at a selected speed which may be onesuitable for printer control directly, or a much `faster speedapproaching the maximum bit-rate capacity of the available telephoneline or channel) is conveyed over the channel 32, manual switch contacts34, and channel 36 to the data-tone transmitter 38 which generates codedaudio tones and applies them to telephone line 10. This audio tonegenerator 38 is a commercially available device known under thetrademark Dataphone and described in the article mentioned above. Thesame remark applies to its companion audio-tone data receiver 40.

The possibility of using a very high recording speed at the receivingend of the line permits the new system to avoid the inherent speedlimits of known punches, and permits the maximum use of the band widthof available channels, since tape recording and reproduction at rates ofas high as 1,000 bits per second, and upward. is technically feasible.

The data bits received at Station #2 out of the datatone device arerecorded for later use or may be conveyed directly to its input-outputprinter, or both simultaneously. It will be understood that directcontrol of the receiving printer is possible only when thebit-transmission rate is at the lower speed suitable for such machinecontrol. Since the station equipments are identical, the receivingoperation can also be followed by referring again to Station #1, withmanual send-receive switch 42 in the opposite position from thatillustrated. That switch operation opens the connection between channelsor conductors 32 and 36 at contacts 34, and closes contacts 44 toconnect the output of data-tone receiver 40 over channels 46, 48 to therecorder 28.

It will be noted that send-receive switch 42 selectively appliesoperating current (via contact 52) from source 50 either to the encoder24 (for recording a tape to be transmitted) or to a decoder 54 (fordecoding from a tape or directly from the line the signals which havebeen received).

Upon conclusion of a received message, indicated by a prearranged signalor otherwise, the tape reproducer 30 is operated to supply reproducedcodes over channel 56 to the decoder 54 which converts them to properform for application via channel 58 to the several control input leadsof the printer character and function operating elements (solenoids) tocause the printer to type out the received message or data.

It is theoretically possible to connect typewriters over telephone orlike circuits without the use of intermediate long-term storage asprovided by the recorder-reproducers 2&-30, but as a practical matterthe system disclosed has important and even essential advantages. Thestorage system provides an optimum high and uniform pulse repetitionrate for transmission purposes, as opposed to the slower and more orless random rate of keyboard operation; it enables the actualtransmission to take place very rapidly when the telephone circuit (andreceiving station) becomes available or free; and it presents thepossibility of editing or correcting known errors in the tape, due tokeyboard mistakes, before actual transmission occurs.

FIG. 2 shows a typical physical arrangement of the equipment at onestation, the same numerals being applied as above. Thus, the typewriteror printer is indicated as a whole by numeral 12 with keyboard 14,printer (type ball) 16, key contacts 18 and keylever operators 20. Theelectronic coder and decoder are shown at 24, S4 in a single cabinet,and the tape recorder-reproducer is indicated by 28-30 with usualrecord-erase head 60, and reproducer `head 62. The tape is arranged tobe fed in either direction, one bit-space at a step, by drives 66 andy60, the reverse motion being controlled from backspace key 70 in amanner to be described below. The data-tone generator and receiver areshown in a common cabinet 38-40.

With the general features and functions above in mind, the detailedaspects of the invention will be more fully comprehended by considering,as though they were separate equipments, the transmitting, receiving andrecordreproduce operations in turn. The transmitting flow diagram ofFIG. 3 will be described first. However, it is to be noted that in theinterest of specificity of disclosure, the selection of a particularprinter (the Selectric) has been made, and that certain operatingfeatures of this particular printer or typewriter require specialoperating features that are not essential to the utility of theinvention in its broader aspect. For example, this particular printerdoes not operate code-permutation contact sets for functions such ascarriage-return, tabulating, case shifting and the like, but merelyprovides a single normally-open contact which the key closes whendepressed. Also, such a function as carrier-return may require a longerinterval to complete than is required for each character imprint.Provisions of the associated equipment to accommodate such peculiaritiesmay or may not be needed when other printers are employed.

Recording Refer now to FIG. 3, which diagrams the signal paths andoperations of the equipment when recording. The operation of anycharacter key of typewriter keyboard 14 operates a particular set ofcircuit contacts peculiar to that character. Since these contactclosures (or openings) occur simultaneously, the keyboard output isessentially parallel coded, which is to be converted to serial-coded forrecording. Also, the contacts do not remain operated for a predictablysufficient duration for reliable conversion to serial form. Accordingly,the contact circuit conductors 302 (part of the group 22 of FIG. 1) areapplied to an interim storage buffer 304 which will ultimately conveythem, in serialized order, through a gate circuit 306 and thence to therecording head of tape recorder 28.

Code spacing A particular contact conductor 308 energized upon anycharacter key operation initiates a counting cycle of a 4- step counter310 which emits four pulses over lead 312 to a pulse former 314 that inturn supplies pulses over lead 31S to the tape forward drive stepper (66in FIG. 2) to move the tape, bringing an unrecorded portion under therecording head and allowing the tape movement to become constant beforerecording the character. Counter 310, on completion of its fourth step,emits a pulse over conductor 316 to the 8-step counter 318, which vialead 319 triggers on gate circuit 306 eight times in succession, thegate thus passing the on or off pulses from the several Hip-Hop stagesof storage 304 to the tape recorder head over lead 26.

Counter 318 also emits eight pulses in synchronism with the gate controlpulses, these pulses being fed over conductor 320 to the tape step pulseformer 322 which causes the tape to advance one code position for eachbit from gate 306. At the same time the clock pulses are fed overconductor 370 to the recording circuits.

The Start Pulse (on line 350) also triggers the Write LTI lll

Monostable" 380 which enables the vrecorder write circuit (line 371) byshifting from a minus voltage to a plus voltage only during the time thecode is being entered.

The last pulse from counter 318 triggers over lead 324 a further 4stepcounter 326 which supplies four additional motor step pulses to the tapedrive from pulse former 314, to move the tape clear 0f the recordinghead after recording of each code group corresponding to one characteror function key operation. The blank 4-bit wide guard band on each sideof a code sequence on the tape ensures that when erasure of codes is tobe performed (as described below), adjacent codes will not be disturbed.In addition, the blank guard space allows the tape to reach a constantstepping speed before actual code recording or reading are performed,and during playback (reading) provides suflicient time for each printingcycle to be completed.

Coding for functions Since, as stated, the function keys of keyboard 14do not operate combinations of contacts, but only make one contact, thetypical function key does two things. First, it initiates the completecounter operation as above described, over conductor 328, and it alsoenergizes a particular input lead of a diode (or equivalent) codingmatrix 330, via conductor 332. The combination of output circuits of thematrix, thus energized, is stored in code storage 304 over a group ofleads symbolized at 334, just as in the case of a character keyoperation. Coding matrixes, as such, are familiar to those skilled inthe datahandling art, and the details thereof are hence not describedherein.

In the special case in which a particular printer operation requires anextended time for its completion, such as is the case for carrier return(and line feed) of a Selectric printer, or the paper-carriage-returnoperation of other typewriters, coding matrix 330 emits a special signalpulse over conductor 336 to a monostable multivibrator 338 having arestoration time greater than the maximum required for such particularextended printeroperating cycle. The aim here is to ensure that sucientblank recorder tape follows such a group, as to allow the receivingprinter, which it will later control, to operate completely before thenext character (or other) operation is called for.

To this end, the pulse from conductor 336 ips the monostable 338 whoseoutput operates relay 340 closing a feedback loop from counter 326 overlead 342 to counter 310 over lead 344. The counters fire one another incontinuous succession, pulsing the pulse former 314 continually untilmultivibrator 338 restores itself in the known way; this steps therecorder tape an extended distance after the function code has beenrecorded thereon.

The time constant of multivibrator 338 is such to allow, for example,three complete cycles of the 16-bit stage array to occur every time acarrier-return function is coded. However, it is to be realized thatunless additional provision is made to suppress the start" pulses thatnormally are generated concurrently with the first recorded bit of acode, during these added cycles of counter stage operation, such a startpulse will be recorded on the tape at the beginning of each of thedesired blank spacesl following the recording of the carrier return codegroup. When these added start pulses are reproduced at the receivingend, they would initiate undesirable machine cycles and result in theprinting or simulating of wrong characters; that is, at least one of theoperating magnets of the receiving printer would be operated.

The supernumerary start pulses, following the actual recording of thecarrier return code, are suppressed by the repeat-inhibit circuit 346,which is essentially a monostable multivibrator that is normally in acondition so that the inverting amplifier 348, which it controls, allowsthe start pulse from the first stage of ring counter 318 to be recordedover the channels 350 and 352 to recorder 7 28. When multivibrator 338is tiipped, as it will be upon operation of the carrier-return key ofthe keyboard, it also enables at 374 the multivibrator 346 which istriggered by the second counter 318 to inhibit 346, which now cuts offthe amplifier 348 (or closes the gate) and thereafter, the start pulsesfrom counter 318 are inhibited from reaching the recorder duringrepeated counter stage cycles. After the multivibrator 338 has restoreditself to normal, the inhibit circuit 346 is also restored, opening thechannel through inverting amplifier 348 and thereafter a start pulsewill again be recorded coincident with the first bit of each subsequentcode group. The multivibrator 346 is inhibited by line 374 during otheroperations so that pulses arriving on line 373 do not trigger it.

Recapitulating the foregoing, operation of keys of the typewriter atStation #1 causes the generation of code groups directly, where the keysoperate a suitable number of switches or switch contacts, or indirectlyvia the coding matrix 330 when the keys call for functions that are notfully coded by the typewriter contacts. In either case, the final codesare put in serial form by the buffer storage 304 and passed, at properlytimed intervals under the control of counter 318, to the tape recorder28. The 4-step counters 310 and 326 provide for additional tape motionbefore and after the actual code-bit recording space, and in the case ofa carirer-return code, the recycling arrangement involving multivibrator338 also provides for added tape movement (free of start bits) followingthe recording of such code.

The code recording on the magnetic tape is not the recording of segmentsof tone frequencies, but rather on a DC basis by the recording of amagnetized condition as differentiated by an unmagnetized condition ofeach successive bit-space on the tape. The reading head of therecorder-reproducer 28, when it scans the recorded bit groups, controlsa suitable amplifier 354 to provide the proper voltage levels forcontrolling the data-tone generator 38 which includes its own audio-tonegenerator in the known manner of operation of such devices. It is thesetones, then, which are transmitted over the telephone channel to thereceiver whose operation will now be described.

Reception of codes When the tape which has been recorded as describedabove is to be employed for transmitting data to the receiving end ofthe channel, it is run through the recorder which is now conditioned toread the tape and generate output pulses corresponding to thoseoriginally recorded. The coded groups of output pulses are spaced asabove described, and the first bit or mark pulse of each group signifiesthe start of a compite code that will ultimately call for operation of acharacter or function operation at a printer, or in a computer or thelike. Two modes of operation are possible; one in which the receivedcode groups directly control a typewriter, such as the inputoutputSelectric mentioned above, having a limited speed of operation; andanother in which the received code groups are recorded at high speed,resulting from the operation of the tape machine at the transmitting endat a much higher speed than it had when it was being encoded. A thirdcombination is possible, in which the codes, bein-g received at the`lower speed, can simultaneously control the reeciving typewriterdirectly, and also be recorded for later use, or retransmission, upon astorage tape.

Turning now to FIG. 4 of the drawings, the arrangement of the equipmentat the receiving end is shown schematically. During the reception ofcodes transmitted from the data-tone unit 38 of FIG. 3, the receivedsignals are applied to the data-tone unit 40, whose output isconstituted by the production of significantly different DC potentiallevels on a pair of output conductors or contacts, these beingsymbolized by the conductor 401. Since these levels recur in time-serialform, they can be used directly to energize the recording head of thetape machine 28 at this end, which of course run at a speed commensuratewith that at which the transmitting machine of FIG. 3 is being operated.Since the system is an asynchronous one, actual synchronism of tapespeeds is not a requirement, the requirement being merely that the markand space voltage levels which constitute the bits not be jammed tooclose together for accurate read-out, and not spaced apart so far thatrecording space is wasted. At the receiving end, a clockcounter 400supplies the equivalent of synchronizing pulses to the tape recorder 28.

When the magnetic tape at the receiving end has recorded the entiremessage, the tape machine 28 is switched to its read-out or reproducecondition, and the tape reinserted for controlling the typewritter atthat end of the circuit. The output voltage levels from the reading headare applied to conductor 402, amplified to suitable amplitude byamplifier 404, and applied to the input of shift register 406, whichconsists of eight fiip-fiop stages interconnected in the usual way toallow each stage to be set individualy in accordance with whether a markor a space condition exists in each position of the tape as it passesover the reading head. The first pulse of each code group constitutesthe start pulse, and is shaped in the start pulser 407 and applied to amonostable multivibrator 408 which acts as a delay circuit to preventreading out of the stage conditions of the shift register before thecode has been completely entered therein. The multivibrator 408 alsofurnishes a pulse to start the counter 409, which thereupon generateseight successive pulses which are respectively delayed by approximatelyone-half of a bit interval in the shift pulse delay circuit 412.

After the complete code group has had time to be entered in the shiftregister, the read-reset multivibrator 410 is energized as describedabove, and its output conditions the read-out circuit 414 whose inputsare connected to the respective stages of the shift register, so thatthe eight bits (mark or space conditions) are applied in time parallel(that is, simultaneously) to the driver amplifiers indicated at 416 andthence to the printer control amplifiers 418. These amplifiers aredesignated as character selectors" at 418, and one such amplifier isconnected to each of the character selecting magnets of the receivingtypewriter.

When the read circuit 414 receives a code calling for a functionaloperation, as distinguished from a character selection, the combinationenergizes the appropriate driver 424 for the energization of thecorresponding functioncontrol magnet of the Selectric typewriter. Inessence, the function-decode circuit is an anding" circuit of sevenstages which provides input to the drivers only when the proper bits fora function are present in the read circuit 414. The function decodingcircuit 420 has an additional function, necessitated by the fact that,as stated above, the Selectric typewritter will go through a machinecycle if any of its magnets are energized for any reason, printing anerroneous character even if only a single bit is present. To preventthis result, the character-selecting operations are inhibited when afunction code is sensed by the function decode circuit 420. An auxiliaryvoltage output of the circuit is applied at 422 to the driver circuits416 when this happens, preventing them from causing outputs from thecharacter selector amplifiers 418.

The proper character or function operation called for by the code groupread from the tape machine 28 has thus been performed, and as theoperate-time of the readreset multivibrator 410 expires, the shiftregister 406 is reset (that is, all of its ip-tiop stages are restoredto a zero or space-bit condition) at 428 in preparation for theconversion of the next serial code-group into parallel form in the sameway.

Parity check FIG. 4 also diagrams the operation of checking each codegroup reproduced from recorder-reproducer 28 at the receiving station togive a signal alarm if the established parity (odd or even number ofmark or on bits in each group) is violated. Such systems enablesingle-bit errors to be detected in a known manner, and the operationwill be described on the basis that the selected code scheme uses an oddnumber of mark bits in each correct code group.

The code pulses from amplifier 404 are applied over lead 430 to theparity-check code delay circuit 432, a monostable circuit having a quasistable period at least equal to the duration of a complete code group atthe bit rate employed. The first or start pulse of each group also firescounter or clock 434 which releases a series of eight sharppositive-going pulses (one for each bit position of a code group) to thel/2-bit delay circuit 436. The latter delivers these pulses to one inputof a combining amplifier 438.

The code pulses from lead 430 are also applied to the polarity invertingamplifier 44|) (since the output pulses of amplifier 404 arenegative-going) which furnishes the second input to combining amplifier438. Due to the 1/2- bit delay of circuit 436, each of the sharp pulsesfrom it falls approximately at the mid-position of each bit interval ofthe code. Since the combining amplifier 438 adds the amplitude of thesharp sampling pulse to that of each code bit (O or -t-), the outputlevel of the combining amplifier rises sharply at the instantcorresponding to the center of each mark bit as indicated at 442. Thiscenter-sampling technique is required for `bit-counting purposes becauseadjacent mark bits in the code group stand shoulder-to-shoulder withoutany separation (the same is true of adjacent space bits); henceindividual mark-bit counting requires sampling at each bit interval.

Clipper 446 passes only the voltage level above the mark-conditionlevel, hence only those spikes (originated by clock 434) which areriding upon mark bits, and not those riding upon space bits. Thesepulses are conveyed over lead 448 to a bistable ilip-op 450 whose output(ip or flop) will thus depend upon whether an odd or even number ofpulses is supplied by clipper 446 during each pulse interval. The ip-op45t] is reset to a specific one of its two states, after each codegroup, from the read-reset monostable multivibrator 452, which thenafter the delay introduced by 432, reads the flipped or floppedcondition thereof resulting from the pulses produced at clipper 446. Ifthe code has kbeen properly received, and an odd number of mark bits ispresent, the flip-hop 450 will have no output at lead 454; if an outputis present, an error will be signalled and/or counted at indicator 456.

Correction of keyboard errors It will be recalled that, as shown in FIG.2, the original magnetic tape record is prepared at the sending end byoperation of a typewriter keyboard having a backspace key 70 which movesthe type carrier in the direction opposite to that used in typing in theusual way. When an erroneous letter or character key is operated, it isdesirable that the incorrect code group recorded on the magnetic tape beerased.

FIG. illustrates how the system of the invention accomplishes thisresult. Since the backspace key 70 is a function key, its operationnormally applies a voltage pulse to coding matrix 330 (of FIG. 3 also)to apply the proper code sequence to recording head 60 over the circuits304, 306 of FIG. 3 which have not been duplicated in FIG. 5 to avoidcomplicating the drawing. In such normal backspace recording operations,the forward stepping drive 66 of the recorder tape would also be stepped16 steps by the counter array 310, 318, 326.

For use when the typewriter carrier is backspaced to correct an error,FIG. 5 shows a correction switch 502 which, in its normal or recordposition, completes the circuit from the backspace key contacts 504 tothe coding matrix over normally-closed contacts 506 of the switch. Inthis condition, also, contacts 508 ofthe switch apply the pulses fromthe counter array to the forward drive stepper 66 of the recorder.

When the correction switch 502 is thrown to its correct" position,contacts 506 open, interrupting the path from contacts 504 to thematrix. However, operation of the backspace key still initiates the l6step cycle of the counter array, over lead 328. The output of thecounter is now directed to the reverse stepper drive 68 of the recorder,due to the closure of -switch contacts 510 and the opening of itscontacts 508. Finally, switch contacts 512 apply a source of erasingvoltage to the erase-record head of the recorder. In practice, therecorder may have a single capstan and ibi-directional motor, only thedirection-control wiring being reversed.

It follows that each operation of the backspace key, when the correctionfeature is in use, the tape back 16 steps and erases its clean of thecorresponding code group. If an error lies several characters back inthe typescript, the backspace key is operated the requisite number oftimes, and the correction switch then restored to its record" position.The correct keys are then operated to record the proper codes upon thetape in the way already described.

FIG. 6 illustrates schematically a typical section of record tape, thebits recorded thereon `being shown as code pulse waveforms merely forclarity. As shown, the typical S-bit sequence consists of a start pulse(always, for example, a "mark pulse), followed by six code-determiningbits which are selectively mark or "space bits, and terminating in aparity-check bit which will be either a mark" or space depending uponwhether the center six bits include an odd or an even number of mark"bits. The 4bit length guard bands on each side of the 8- bit sequenceare also shown.

While the invention has been described and illustrated herein inconnection with a particular system embodiment chosen for purposes ofclear exposition, it will be apparent to those skilled in the art thatvarious changes and rearrangements can be effected without departingfrom the invention, and it is intended to include herein all suchmodifications as fall within the scope of the appended claims.

I claim:

l. In a data transmitting system:

(a) an input keyboard having both character selecting keys and functionselecting keys,

(b) a multiple contact set controlled by each character selecting keyand a single contact set controlled by each function key,

(c) a coding matrix controlled by each of said single contact sets,

(d) a coding storage device for sequentially storing codes selected byeither said multiple contact sets or said coding matrices,

(e) a signal recorder, and

(f) gate means for conveying coding sequences stored in said storagemeans to said signal recorder,

(g) a multistage counter for emitting a timed sequence of pulses uponeach energization of its input,

(h) means controlled by each of said keys for energizing the input ofsaid counter,

(i) a counter feedback circuit from the output of said counter to itsinput to produce recycling of its stages,

(j) means controlled by one of said coding matrices for completing saidfeedback circuit for a predetermined period upon actuation of thecorresponding function selecting key,

(k) means controlled by the output of said counter for advancing therecording medium of vsaid signal recorder,

(l) means controlled by an intermediate stage of said counter foroperating said gate means to convey the stored code sequence to saidsignal recorder during an intermediate portion of its recording mediummotion, and

(m) means for erasing individual characters on the magnetic tape byyback-spacing from the keyboard.

2. In a data transmitting system:

(a) an input keyboard having both character selecting keys and functionselecting keys,

(b) a contact set controlled by each character selecting key and acontact set controlled by each function key,

(c) a code storage device for sequentially storing codes selected bysaid contact sets,

(d) a signal recorder, and

(e) gate means for conveying coding sequences stored in said storagemeans to said signal recorder,

(f) multistage counter for emitting a timed sequence of pulses upon eachenergization of its input,

(g) means controlled by each of said keys for energizing the input ofsaid counter,

(h) a counter feedback circuit from the output of said counter to itsinput to produce recycling of its stages,

(i) means controlled by one of said function key contact sets forcompleting said feedback circuit for a predetermined period uponactuation of the corresponding function selecting key,

(j) means controlled by the output of said counter for advancing therecording medium of said signal recorder, and

(k) means controlled by an intermediate stage of said counter foroperating said gate means to convey the stored code sequence to saidsignal recorder during an intermediate portion of its recording mediummotion.

3. A data transmiting system in accordance with claim 2, in which thesaid predetermined period has a duration several times as long as therecycling time of said counter.

4. In a data transmitting system:

(a) an input keyboard having selecting keys,

(b) a contact set controlled by each of said selecting keys,

(c) a code storage device for sequentially storing codes selected bysaid contact sets,

(d) a signal recorder, and

(e) gate means for conveying coding sequences stored in said storagemeans to said signal recorder,

(f) a multistage counter for emitting a timed sequence of pulses uponeach energization of its input,

(g) means controlled by each of said keys for encrgizing the input ofsaid counter,

(h) means controlled by the output of said counter for advancing therecording medium of said signal recorder,

(i) means controlled by an intermediate stage of said counter foroperating said gate means to convey the stored code sequence to saidsignal recorder during frequency telephone circuit, at least one of saidterminal stations including:

(a) a moving carrier typewriter having (l) a selecting keyboardincluding a carrier backspacing control key, (2) code-group generatinginstrumentalities connected for control by said keyboard, and (3)circuitselecting contacts operated by said keyboard;

(b) a selective-frequency audio tone generator connected to saidtelephone circuit for applying thereto coded tones corresponding to theenergization of respective combinations of input conductors of saidgenerator,

(c) a stepping-type digital code magnetic tape recorderreproducerincluding a bidirectional tape feed drive, input terminals for therecording of signals, and output terminals for the reproduction ofrecorded signals,

(d) electronic signal processing circuits including a coding matrixconnected to said circuit-selecting contacts for selectively energizingthe input terminals of said recorder-reproducer, and forforward-energizing its tape feed drive, in accordance with thesequential operations of said keyboard,

(e) a contact set operated by said backspacing control key, and amanually-operable control switch having one position in which it extendsa first control circuit from said contact set to saidrecorder-reproducer to backward-energize its tape feed drive for eachoperation of said backspacing control key to allow correction of codegroups erroneously recorded on the magnetic tape,

(f) said manually-operable control switch having another position inwhich it interrupts said first control circuit and extends a secondcontrol circuit from said contact set to (l) said recorder-reproducer toforward-energize its tape feed drive, and to (2) said coding matrix soas to record a backspace code group on said magnetic tape, for eachoperation of said backspacing control key.

6. A system in accordance with claim 5, and switch means for selectivelyconnecting either the input terminals of said recorder-reproducer tosaid telephone circuit, or the output terminals of saidrecorder-reproducer to said tone generator.

No references cited.

ROBERT L. GRIFFIN, Primary Examiner.

W. S. FROMMER, Assistant Examiner.

